1. Field of the Invention
The present invention relates to the process of photolithography used in the manufacturing of semiconductor devices and the like. More particularly, the present invention relates to a photo mask and to a method of manufacturing the same.
2. Description of the Related Art
Process margins in the manufacturing of semiconductor devices are becoming smaller and smaller as the design rule of the devices has decreased to meet the demand for more highly integrated devices. One semiconductor manufacturing process that is being refined is photolithography. In photolithography, light of a given wavelength is directed from a light source onto a substrate through a photo mask such that an image of the photo mask is projected onto the substrate. The image is developed and exposed to form a pattern on the substrate. Conventionally, a KrF light source has been widely used as the light source, and a binary mask (BM) has been used as the photo mask. Binary masks comprise a quartz substrate having a chrome pattern thereon.
Recently, an ArF light source or an F2 light source has been proposed as a substitute for a conventional KrF light source as a means of attaining a smaller margin for the photolithography process. However, many problems exist in the mass-production of ArF light sources and the like. Therefore, Resolution Enhancement Technologies (RETs) are actively being developed which enable fine patterns to be formed using existing light sources, such as the conventional KrF light sources.
Among these RETs is a photolithographic process that uses a phase shift mask (PSM) instead of the conventional binary mask (BM). A PSM induces a phase shift in the light being transmitted therethrough to enhance the resolution of the photolithograph process. A PSM may be classified as an ALT-PSM (alternating-PSM), a rim-shift-PSM, an attenuated PSM (hereinafter, referred to as attPSM) or a half-tone PSM. An ALT-PSM (alternating-PSM) comprises a quartz substrate, and phase shifters in the form of film portions or recesses spaced from one another by a predetermined distance on a light-transmitting area of the quartz substrate. A rim-shift-PSM comprises a light-blocking pattern made of chrome, and phase shifters in the form of film patterns formed on the light-blocking pattern. An attPSM or a half-tone PSM comprises a half-tone film instead of the light-blocking pattern or a film of Mo applied to a conventional BM.
Currently, the attPSM is the most widely used type of PSM in photolithography. However, the film constituting the phase shifter of the attPSM has a transmissibility of only 5 to 20%. Therefore, a so-called side lobe is produced. This problem can be prevented by employing an additional light-blocking pattern. Nonetheless, such a light-blocking pattern requires an additional process that increases the Turn Around Time (TAT) for manufacturing the mask. Also, an Mo film causes a haze to develop on the mask. The haze is not present during the first few series of processes, but abruptly develops over the entire surface of the attPSM once the processes have been repeated a number of times, even to the point where the process yield is 0%. Therefore, an attPSM must be periodically cleaned to prevent haze. However, it is impossible to completely prevent a haze from eventually developing on an attPSM.
For these reasons, current studies into improving the resolution in photolithography have centered around the use conventional BMs made of quartz and chrome. Among the photo masks being studied in this respect are polarized masks (referred to hereinafter as “PMs”). A polarized mask (PM) has a chrome pattern that is thicker than that of a conventional BM or light-transmitting areas that are narrower than those of a conventional BM. The PM enhances the resolution of the photolithography process by transmitting the incident light, made up of TEM (Transverse Electric and Magnetic) waves, at a relatively high polarization ratio (the ratio of the intensity of the transverse electric (TE) waves to the intensity of the transverse magnetic (TM) waves). Because the relatively thick chrome pattern or the closely spaced portions of the chrome pattern of a PM absorb the TM waves very well, the chrome pattern is also referred to as an absorbing layer in a PM.
FIG. 1A shows the change in ratio of the intensity of TE waves to the intensity of TM waves according to changes in the thickness of the absorbing layer. Referring to FIG. 1A, as the absorbing layer becomes thicker, the ratio increases. Accordingly, the absorbing layer should be as thick as possible to enhance the resolution of the photolithography process as much as possible. However, making the absorbing layer thick complicates the manufacturing process of the photo mask.
FIG. 1B shows the intensity of the TM waves, the intensity of the TE waves and the polarization ratio according to changes in the pitch of the light-transmitting area. Referring to FIG. 1B, the polarization ratio continuously increases as the pitch of the light-transmitting area decreases, and sharply decreases when the polarization ratio exceeds a critical value (96.5 nm in FIG. 1B). Accordingly, the pitch of the light-transmitting area should be set at the critical value for the photo mask to enhance the resolution of the photolithography process to the maximum extent possible. However, the pitch of the light-transmitting area is limited by the design rule and/or a shape of the pattern to be formed on the substrate. Thus, it is difficult to use the critical value in designing PMs for mass-production.